During the past year, we have continued our investigation of the structure of chromatin, the DNA-protein complex found in eukaryotic nuclei. We find that nucleosomes, the fundamental chromatin subunits, can be reconstituted using histone H3 that has been dimerized by cross-linking of the single sulfhydryl groups of the cysteines at position 110. The physical properties of such nucleosomes are so far indistinguishable from those observed for nucleosomes containing reduced H3 molecules. If, as seems likely, the nucleosome has a dyad axis, the disulfide bridge must lie on the axis. Using cloned, sequenced DNA fragments 140 base pairs in length, we have reconstituted nucleosomes, shown that a well-defined phased complex is obtained, and begun to examine the accessibility of the individual nucleotides in the complex to a variety of chemical probes. We also have continued our studies of transcriptionally active genes in chromatin, notably the globin genes of avian reticulocytes. We have developed a new rapid method for the purification of closed circular DNA plasmids, which makes possible the convenient preparation of large quantities of cloned DNA containing the globin gene for use in our transcriptional studies.